The major factors which influence the flow properties of whole blood are the concentration, deformability and geometry of the red blood cells as well as the extent of cellular aggregation present and the composition of the plasma. The research project is focussed on an examination of the relative influence of parameters such as cell geometry, surface charge and deformability in determining the macroscopic flow behavior of red cell suspensions or whole blood. The rheological behavior of normal and aldehyde-fixed human red blood cells at comparable volume concentrations suspended in media containing various concentrations of dextrans or plasma proteins will be examined over a wide range of shear rates by rotational viscometry. The degree of cellular aggregation present under comparable conditions will be estimated by light microscopy. The surface charge of the cells will be obtained by cell electrophoresis. The deformability of cells will be measured by centrifugal packing coupled with estimates of the trapped fluid volume. Aldehyde-fixation procedures are being established for human red blood cells which satisfy both electrokinetic and rheological criteria as regards reproducibility from batch to batch of cells. The relative zeta potential of formaldehyde-fixed red cells suspended in dextran has been found to be lower than for normal erythrocytes indicating that the adsorption of dextran is less. A parameter termed the relative viscosity ratio has been found to be a satisfactory rheological index of aggregation for both normal and formaldehyde-fixed red blood cell systems. It is planned to extend the studies to whole blood so that a comparison may be made between the behavior of blood samples from normal subjects and those from patients with a variety of diseases in which one or more of the parameters under study are altered.